1. Field
The present disclosure relates to a method and a system for color stability in an image printing system.
2. Description of Related Art
An electrophotographic, or xerographic, image printing system employs an image transfer surface, such as a photoreceptor drum or belt, which is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the image transfer surface is then exposed to a light image of an original document being reproduced. Exposure of the charged image transfer surface selectively discharges the charge thereon in the irradiated areas to record an electrostatic latent image on the image transfer surface corresponding to the image contained within the original document. The location of the electrical charge forming the latent image is usually optically controlled. More specifically, in a digital xerographic system, the formation of the latent image is controlled by a raster output scanning device, usually a laser or LED source.
After the electrostatic latent image is recorded on the image transfer surface, the latent image is developed by bringing a developer material into contact therewith. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the image transfer surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to an output media, such as sheets, paper or other substrate sheets, using pressure and heat to fuse the toner image to the output media to form a print.
The image printing system generally has two important dimensions: a process (or a slow scan) direction and a cross-process (or a fast scan) direction. The direction in which an image transfer surface moves is referred to as the process (or the slow scan) direction, and the direction perpendicular to the process (or the slow scan) direction is referred to as the cross-process (or the fast scan) direction.
The image printing systems may produce color prints using a plurality of stations. Each station has a charging device for charging the image transfer surface, an exposing device for selectively illuminating the charged portions of the image transfer surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to an output media. The transferred multicolor image is then permanently fused to the output media forming the color print.
Color stability continues to be a major print quality issue for the image printing systems. Colors may vary within a page, from page-to-page, from job-to-job, from day-to-day, and from machine-to-machine. Further, determining root causes of color stability problems can be a trial and error procedure for customers and service engineers. Both customers and service engineers often replace many parts of the image printing system searching for the fix to a color stability problem because it can be difficult to determine the root causes of the color stability problem. Because the color stability problems are difficult to isolate, many parts of the image printing system that come back from the field do not exhibit the color stability problem that they were replaced to fix. This results in wastage and additional run cost for the image printing system.
An additional problem that arises is that color stability noises can be both structured and unstructured. The most prominent structured noise is process direction banding, while unstructured noises are those that exhibit statistical independence (are “random”).
Banding generally refers to periodic defects on an image caused by a one-dimensional density variation in the process (slow scan) direction. Bands can result due to many different types of variations within components and/or subsystems, such as roll run out (variations in roll or drum diameter) in a developer roll or photoreceptor drum, wobble in the polygon mirror of the laser raster optical scanner (ROS), and the like. Various sources of banding exist in the image printing system and the frequencies of these sources may be known based on the mechanical design of the image printing system.
Distinguishing between the two types of noises (i.e., structured and unstructured) is important because the solution approach to each type of these color stability noises is quite different. For example, banding from multiple simultaneous sources can appear to be unstructured (chaotic), while actually being deterministic.